Communication systems typically include a plurality of dispatch consoles and communication units, such as mobile or portable radio units, that are geographically distributed among various base sites and console sites. The communication units wirelessly communicate with the base sites and each other, and are often logically divided into various talkgroups. Communication systems may be organized as trunked systems, where a plurality of radio frequency (RF) communication resources are allocated amongst multiple users or groups by assigning the base sites and RF channels within a coverage area on a call-by-call basis, or as conventional (non-trunked) systems where RF communication resources are dedicated to one or more users or groups. In trunked systems, or in mixed trunked and conventional systems, there is usually provided a central controller/server (sometimes called a “zone controller”) for allocating RF communication resources among a group of sites. The zone controller may reside within a single device or multiple devices and may be located at a fixed equipment site or may be distributed among the base sites.
Calls among the communication units are typically of the dispatch type, or better known as push-to-talk (PTT). Each time a communication unit is keyed to begin a call, the call is assigned to an available wireless channel. The originating communication unit begins transmitting on the assigned channel and continues to transmit on the assigned channel until the communication unit has been dekeyed by the user. Other communication units in an associated talkgroup also switch to the assigned channel and begin listening to the call.
To ensure that communications units are utilizing the correct channel, a network access code is also assigned (either statically or dynamically) for each channel. The network access code is encoded and transmitted along with the voice audio information. The network access code is often referred to as a color code. Each base site is then typically configured to validate the color code before broadcasting the call to other communication units to ensure that the source of the incoming call is transmitting on the correct channel. Each receiving communication unit is typically also configured to receive and validate the color code for each call.
One set of industry standards commonly used for communication systems is referred to as Project 25, developed by the Association of Public Communications Officials (APCO). Currently deployed Project 25 systems, generally referred to as Phase 1 systems, utilize Frequency Division Multiple Access (FDMA) modulation for transmitting call information. In such systems, the color code is typically transmitted in one continuous data block prior to the start of voice audio information blocks. As such, the color code can be acquired and validated quickly in a single step. This operation, however, cannot be utilized in a Time Division Multiple Access (TDMA) solution in which the color code must be distributed across multiple TDMA bursts since the color code cannot be properly validated in a single step. Thus, validating a distributed color code in a single burst, as required in a TDMA solution, could result in audio that was improperly validated causing possible audio anomalies, such as incorrectly muted audio.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help improve the understanding of various embodiments of the present disclosure. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are not often depicted in order to facilitate a less obstructed view of these various embodiments of the present disclosure. It will be further appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein are to be defined with respect to their corresponding respective areas of inquiry and study except where specific meaning have otherwise been set forth herein.